1. Field of the Invention
The present invention relates to an ink recycling type inkjet printer that performs printing by discharging a recycling ink from an ink head, and more particularly, to a technology of suppressing a change in pressure in a nozzle of an ink head within a predetermined range at the time of starting the recycling of ink.
2. Description of the Related Art
It is known from the related art that an inkjet printer performs printing on a recording medium, such as printing paper, by discharging ink from a nozzle of an ink head. Further, as the inkjet printer, there is a an ink recycling type inkjet printer that continuously recycles ink at the time of printing so as to cool the ink or remove wastes in an ink channel.
The ink recycling type inkjet printer includes an ink recycling path for recycling ink. A downstream side and an upstream side of the ink recycling path are installed with ink tanks through the ink head. Further, a pump, which applies pressure so as to recycle ink, is installed between the ink tanks.
Further, a connection position between the ink recycling path and the ink head is installed with an ink supplying port that supplies ink to the inside of the ink head and an ink discharging port that discharges ink from the inside of the ink head.
Further, the ink head used for the ink recycling type inkjet printer uses a structure where ink can be recycled therein. As one example of the ink head in an ink recycling structure, there is a share mode ink head as shown in FIG. 5. The share mode ink head of FIG. 5 is configured wherein an ink supplying hole 2 and an ink discharging hole 3 are installed inside an ink head 1. The ink supplying hole 2 and the ink discharging hole 3 are communicated with an ink supplying port 4 and an ink discharging port 5, which are installed at an outer side of the ink head 1. Moreover, plural ink chambers 7, which are partitioned by plural partition walls 6 configured of a piezoelectric element are installed between the ink supplying hole 2 and the ink discharging hole 3.
In the share mode ink head 1 shown in FIG. 5, as shown by an arrow in FIG. 5, ink is supplied from the ink supplying port 4 to the inside of the ink head 1 and then flows in each of the ink chambers 7 from the ink supplying hole 2 through an ink supplying channel (not shown) in the ink head 1. Although a portion of the ink, which flows in the ink chamber 7, is discharged from a nozzle 8 that is installed at an approximate center within the ink chamber 7, ink, which is not discharged from the nozzle 8, flows in the ink discharging hole 3. Thereafter, the ink, which flows in the ink discharging hole 3, is discharged from the ink discharging port S to the ink recycling path through the ink discharging channel (not shown) and recycles back the ink recycling path.
Further, the ink head of the ink recycling structure as described above is also described, for example, in JP-A-2006-88575 and the like.
When a channel resistance from the ink supplying port 4 to the nozzle 8 is approximately the same as a channel resistance from the nozzle 8 to the ink discharging port 5, if pressure Pi is applied to the ink supplying port 4 and pressure Po is applied to the ink discharging port 5 at the time of recycling the ink (at the time of printing), a pressure Pn of the nozzle 8 becomes Pn=(Pi+Po)/2 in the ink head of the ink recycling structure such as the share mode ink head 1 shown in FIG. 5.
At this time, when the pressure Pn of the nozzle 8 is a positive pressure larger than 4r/d [Pa] (wherein r is surface tension of the ink, and d is the diameter of nozzle 8), a meniscus of the nozzle 8 breaks, such that ink is leaked from the nozzle 8. To the contrary, when the pressure Pn of the nozzle 8 is a negative pressure larger than −4γ/d [Pa] (wherein γ is the surface tension of the ink and d is the diameter of the nozzle 8), the meniscus of the nozzle 8 breaks, such that bubbles are inhaled from the nozzle 8.
For this reason, when the ink head in the ink recycling structure is used, the pressure in the ink supplying port 4 and the ink discharging port 5 is controlled at the time of recycling the ink, such that the pressure in the nozzle 8 is controlled to be in the range of the above-mentioned equation.
However, when the inkjet printer is not used over a long period of time, that is, as shown in FIG. 6, when the recycling of ink remains stopped for a long time, the viscosity of the ink in the vicinity of the nozzle 8 of the ink head 1 is increased.
Generally, the inkjet printer is needed to recycle ink for a predetermined time (warm up) so as to be able to move from a standby state where the recycling of ink stops to a state where actual printing starts by recycling ink. However, if the inkjet printer of the related art recycles the ink like the above-mentioned state, as shown in FIG. 6, when the viscosity of the ink 9a is increased at the upstream side of the nozzle 8, the channel resistance of the upstream side becomes larger than the nozzle 8, such that a large negative pressure is applied to the nozzle 8. As a result, the meniscus breaks, causing air bubbles to be inhaled from the nozzle 8. Further, when the viscosity of ink 9b is increased at the downstream side of the nozzle 8, the channel resistance of the downstream side becomes larger than the nozzle 8, such that a large positive pressure is applied to the nozzle 8. As a result, the meniscus breaks, causing the ink to be leaked from the nozzle 8.